Determing Alternator

[ice00 10+ year member]

how do u tell if ur alternator will work with ur system?...how do u determine if ur alternator will need to be replaced...??

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[mattj 5,000+ posts]

Obviously it will have to be able to fit, and it will have to keep up with the draw thats being pulled. Add the amperage your amps pull and add about 30 for what your car usually needs and thats about the size you should get.

 

[farbs93 10+ year member]

how do u know how many amps an amp pulls if it tells ur in watts. I have 2 500/1 JL Audio Amps and 2 300/2 JL Audio Amps which is 1600 watts rms @ 4 ohms. How many amps is that drawing?

 

[n2audio 5,000+ posts]

what amps CAN pull and what they DO pull are very different. Playing test tones your set-up can probably draw - say, 140A or so?? But music dynamics result in current demands that are nowhere near that of a test tone. About a third is the general rule, and that's still assuming full volume - or close to it. At low/moderate volmes it would probably be significantly less than 1/3.

Based on your set-up, unless you drive a Ford Festiva or a Subaru Justy, or drive around playing test tones at full volume I would be surprised if your stock electrical system will need any mods.

 

[Topcat 10+ year member]

here's a cheap easy way to estimate your systems requirements. First - add all the fuse ratings of all your amplifiers. Then divide the total RMS watts by Total Peak watts and multiply this figure times your total fuse rating. This is the approximate current draw for your sound system. For example, you have three(3) amplifiers, Amp 1 has a 100a fuse and is 1000 watts RMS and 1200 watts peak. Amp 2 is 150 watts RMS and 300 watts peak with a 40a fuse. Amp 3 is 75 watts and 150 watts peak and a 40a fuse. Adding the fuse ratings gives you 180 amps. Total RMS watts will be 1225 watts. Total peak watts wil be 1650 watts. so the math kinda looks like this: 1225 / 1650 = .742 x 180 = 133.56 amps. Another good way to estimate your system current draw is to approximate 85 amps per 1000 watt.

 

[n2audio 5,000+ posts]

here's a cheap easy way to estimate your systems requirements. First - add all the fuse ratings of all your amplifiers. Then divide the total RMS watts by Total Peak watts and multiply this figure times your total fuse rating. This is the approximate current draw for your sound system.

Wow - I don't know where you learned that at - but at best it's nothing more than a bad joke.

Maybe this will help clear things up a little bit. A very thorough post regarding high power systems. The original question concerned a 4000 watt set up.

Originally posted by Richard Clark:[QB] guys the math is really very siimple------4000 watts is 4000 watts and watts is always volts x amps-----so in a 12 volt system it would be 333 amps and at 15 volts it would be 266 amps--------now if we have to pass that wattage through a lossy system (like an amplifier) we have to account for efficiency losses and add a percentage----ie if the efficiency is 50% we have to double our starting amperage since the voltage is still the same------so a 50% lossy device outputing 4000 watts would draw 8000 watts-------at 13 volts this would be about 615 amps--------class AB amps range about 50-60% at full output and class D amps range about 60-80% at full output------NOW for the biggest factor-------if we are playing music we can automatically divide these numbers by 1/3 or more for short term values due to the crest factor of music and for long term values we can divide by even more-----there are pauses between songs and at the end of the disc where the current demands are VERY low and these affect the long term average quite a bit-----and---sooner or later you are bound to ride by a cop and might have to turn it down a bit-----its not likely that a long term value of more than 10% can be needed for actual power requirements in a real system-------so for that 4000 watt system most any stock alternator will do just fine since it is a rare system that requires more than 50 CONTINUOUS LONG TERM AMPS.............RC [/QB]